Phase separation is a technique in which controlled separation of a polymer solution into two phases is used to form a membrane structure. Different forms of phase separation include thermal separation, immersion or coagulation precipitation, vapor phase separation, and liquid-liquid separation. In thermal phase separation, the separation occurs when a homogeneous polymer solution is cooled, resulting in the formation of polymer-rich and polymer-lean phases. In immersion precipitation, a polymer solution is immersed in a liquid non-solvent bath. In this case, phase separation occurs as the solvent is exchanged for the non-solvent liquid. In vapor phase separation, the separation is induced by penetration of a non-solvent vapor into a polymer solution. Finally, in liquid-liquid phase separation: the polymer is dissolved in a mixture of a volatile solvent and a less volatile non-solvent (i.e. a polymer-solvent/non-solvent mixture). During solvent evaporation, polymer solubility decreases, and phase separation occurs.
Phase separation is a process that can be used to produce polymer membranes or constructs that can be used as engineered scaffolds or vascular grafts, as well as coverings/grafts for vascular stents. However, all of the forms of phase separation described above result in a construct or membrane having a single uniform structure. The single uniform structure of the membrane arises from the membrane being prepared using a single liquid mixture having one polymer-solvent/non-solvent ratio. In addition, the preparation of liquid-liquid phase separated constructs is further complicated by a) the ongoing separation of the polymer-solvent/non-solvent mixture during the process of casting and b) the need to prevent the formation of an immersion or coagulation precipitate when adding the non-solvent to the polymer-solvent mixture.
Accordingly, there exists a desire to provide vascular grafts and tissue engineering scaffolds that have variable porosity and variable surface properties. More particularly, there exists a desire for the development of a manufacturing or fabrication process for polymer constructs that would allow for such variable properties in different regions of the polymer membrane structure of the constructs.